Machine, preferably a vacuum pump, with magnetic bearings

ABSTRACT

The invention relates to a machine ( 1 ), comprising a stator and a rotor ( 2 ) which is mounted with radially stable and axially unstable magnetic bearings ( 3, 4 ). Said magnetic bearings ( 3, 4 ) each consist of concentrically arranged magnetic ring sets ( 5, 6; 7, 8 ) in which the stationary magnetic ring set ( 5  or  7 ) is situated inside and the rotating magnetic ring set ( 6  or  8 ) is situated outside. The machine also comprises means ( 21, 23, 24, 27 ) for regulating the axial position of the rotor ( 2 ). The aim of the invention is to simplify a machine of this type. To this end, one ( 4 ) of the two magnetic bearings ( 3, 4 ) is itself equipped with the axial regulation means and at least one coil ( 23 ) controlled by a position sensor ( 21 ) and pole components ( 24 ) surround the outer magnetic ring set ( 8 ) of the axially regulated bearing ( 4 ).

[0001] The present invention relates to a machine, comprising a statorand a rotor which is mounted with radially stable and axially unstablemagnetic bearings whereby the magnetic bearings each consist ofconcentrically arranged magnetic ring sets in which the stationarymagnetic ring set is situated inside and the rotating magnetic ring setis situated outside, as well as means for regulating the axial positionof the rotor. The magnetic bearing shall be so designed that it isespecially suited for suspending rotors in blowers or vacuum pumps,preferably drag vacuum pumps operating at high speed.

[0002] The employment of magnetic bearings in turbomolecular vacuumpumps is known (c.f. EP 414 127 A1) and is well-proven. The documentmentioned discloses magnetic bearings consisting of engaging sets ofmagnetic stator and rotor magnetic rings. In the instance of magneticbearings of this kind and the thus equipped machines, assembly is muchinvolved. Moreover, changes in the length of the rotor due totemperature loads give rise to bearing problems.

[0003] From DE-A 38 18 556 a machine having the characteristics of theaforementioned kind is known. This is a turbomolecular vacuum pump withtwo passive radially stable and axially unstable magnetic bearings. Themeans of controlling the axial position of the rotor consist on the onehand of selecting the operating point for the magnetic bearings suchthat it is shifted with reference to the unstable point in the directionof the high-vacuum side, and on the other hand that a lifting systemwhich is independent of the magnetic bearings be provided, said liftingsystem continually compensating the axial forces arising from the shiftin the operating point. In the instance of a solution of this kind bothcontrol and also the related necessary means are involved.

[0004] It is the task of the present invention to create a machinehaving the characteristics detailed above which compared tostate-of-the-art solutions is more simple.

[0005] This task is solved through the characterising features of thepatent claims.

[0006] In the instance of a machine in accordance with the presentinvention, engaging magnetic ring sets are no longer present so thatmounting complexity is reduced. A lifting system being independent ofthe magnetic bearings to control the axial position of the rotor is nolonger required. In all, the bearing is composed of fewer differentcomponents so that production, warehousing and logistics are simplified.

[0007] Further advantages and details of the present invention shall beexplained with reference to¹⁾ drawing FIGS. 1 to 10.

[0008] Depicted are in

[0009] drawing FIGS. 1 and 2 a schematic representation of machines withrotors which are each supported by a magnetic bearing designed inaccordance with the present invention,

[0010] drawing FIG. 3 a turbomolecular/molecular vacuum pump equippedwith a bearing in accordance with the present invention,

[0011] drawing FIGS. 4 to 7 partial sectional views through magneticbearings in accordance with the present invention with differentlydesigned means for axial control and

[0012] drawing FIGS. 8 to 10 examples for embodiments of the magneticbearings with damping means.

[0013] In the machines 1 depicted schematically in drawing FIGS. 1 and2, the rotating system 2 is suspended in two magnetic bearings 3, 4.Each magnetic bearing 3, 4 consists of two magnetic-ring sets 5, 6(bearing 3) and 7, 8 (bearing 4) respectively. The inner ring set 5, 7in each instance is mounted firmly, the outer ring sets 6, 8 which ineach instance encompass the respective inner ring set concentrically andwithout making contact (slot 9) are components of the rotating system 2.The design is in all rotationally symmetric. A drive motor is notdepicted.

[0014] The rotating system 2 is equipped at both face sides with centralrecesses 11, 12. The walls of these recesses form the receptacles 13, 14for the rotating magnetic-ring sets 6, 8. Receptacle 14 is a pipe-shapedreinforcement made of non-magnetizable material, carbon fibre reinforcedplastic, for example, which is fitted preferably by means of a press-fitseat at the rotating system 2. A section of the reinforcement 14encompassing the recess 12 carries on its inside the magnetic-ring set8.

[0015] Stationary carriers 15, 16 with receptacles 17, 18 for thestationary magnetic-ring sets 5, 7 project into the recesses 11, 12 insuch a manner that the outer ring sets 6, 8 concentrically encompass theinner sets 5, 6. In the drawing figures each of the lower carriers 16have a central bore 19 for a shaft end 20 of the rotating system 2, theface side of said shaft end being assigned to an axial sensor 21.

[0016] The axial sensor 21 is part of the means for axially controllingthe magnetic bearing 4. One or several coils 23 each with an U-shapedyoke 24 open in the direction of the ring set 8, generate the magneticfields indicated by the dashed lines and arrows 25. In drawing FIGS. 1and 2 in each instance two coils 23 are provided encompassing the ringset 8. Their yoke components 24 are separated by a spacing disk 26 madeof non-ferrite material.

[0017] A controller 27 serves the purpose of controlling the coils resp.the magnetic fields generated by the coils 23 depending on the signaloutput by the sensor 21. In the slot 28 located in each instance betweenthe outer rotating rings sets 6, 8 and the coils 23, respectively theface side of the limbs of the yoke components 24, the magnetic forcesserving the purpose of axial control become effective.

[0018] The ring sets 5 to 8 consist each of rings magnetized in theaxial direction arranged with changing poles (as indicated by way of anexample for bearing 3 in accordance with drawing FIG. 1) so that thering sets 5, 6 resp. 7, 8 of the magnetic bearings 3, 4 repel eachother. Preferably, so many outer and inner pairs of rings are providedthat each of the magnetic-ring sets has at both its ends the samepolarity. In the solution in accordance with drawing FIG. 1 the ringsets 5, 6 resp. 7, 8 each form two cylinders arranged concentricallywith respect to each other. The dimensions of the magnetic-ring sets 5,7 resp, 6, 8 are preferably identical in each instance. In the solutionin accordance with drawing FIG. 2 the diameters of the circumferentialsurfaces of the rings of both ring sets 5, 6 resp. 7, 8 of the bearings3, 4 facing each other change in distinct steps (in the same direction),so that also the slot 9 is step-shaped. Also the slot 28 in bearing 4may (deviating from what is depicted in drawing FIG. 2) also bestep-shaped.

[0019] In the upper bearing 3 the cross section of the rotating magnetcan be maintained smaller compared to bearing 4. This saves costs forthe magnetic material.

[0020] In bearing 4 it is required that the slot 28 between the polesurfaces of the yoke components and the magnets, which are held in placethroughout the constant inside diameter of the carbon fibre reinforcedplastic tube, be kept small so that the axial bearing can be effectiveon the magnets.

[0021] The rings of the magnetic-ring sets 5 to 8 are held firmly inplace in their receptacles 13, 14, 17, 18. Annular spacing discs 31 madeof non-ferric materials rest flush against the two face sides of eachmagnet ring so that the magnetic forces become effective preferably inthe slots 9 and 28 respectively. If the material of the annular spacingdiscs 31 has in addition good electrical conducting properties (copperfor example) damping of the rotor movements is already attained by this.

[0022] In addition to the spacing disks, the facing surfaces of therings of the ring sets may be encapsulated so as to protect the magneticmaterials against aggressive gases (for example, hydrogen in dragpumps). As an example, stepped sleeves 32, 33 for the in each instancestationary ring sets 5, 7 are depicted in drawing FIG. 2. At the side ofthe ring sets they are joined to the related receptacles in a gas-tightmanner, welded, for example.

[0023] Preferably the inner and outer rings of the ring sets 5, 6 resp.7, 8 are arranged in pairs. To the end of improving axial control it maybe expedient to add to the outer rotating ring set 8 of the axiallyactive magnetic bearing 4, further rings. Variants of this kind aredepicted in drawing FIGS. 1 and 2. The ring set 8 has two more ringscompared to ring set 7. The two outer rings, designated as 29, have beenadded to the set 8. These may be soft ferric rings; preferably, however,two further magnetic rings are added.

[0024] In the machine 1, a turbomolecular/molecular pump depicted indrawing FIG. 3, stator blades 37 are fitted in the casing 35 with theconnecting flange 36. The magnetically suspended rotor 2 carries rotorblades 38 revolving between the stator blades 37, said rotor bladesproviding the pumping action for the gases. Pump 1 is a compound pump.The section equipped with blades is followed by a molecular pumpingsection 39.

[0025] The rotor 2 is suspended in both magnetic bearings 3 and 4. Themagnetic bearing 3 is located at the high-vacuum side. The carrier 15 ofthe stationary magnetic-ring set 5 with its receptacle 17 is part of abearing star 41.

[0026] The magnetic bearing 4 is located at the fore-vacuum side of thepump 1. Both bearings have approximately the same stiffness. The centreof gravity of the rotating system 2 is designated as 42.

[0027] The pump 1 is equipped with emergency bearings or touchdownbearings 44, 45. The touchdown bearing 44 at the high-vacuum side islocated in a recess within the rotor 11. The touchdown 45 bearing at thefore-vacuum side is located under the magnetic bearing 4 between shaftend 20 and the stationary carrier 16.

[0028] As the drive motor 46, a high-frequency motor with stator 47 andarmature 48 is provided. On the side of the stator there is furthermoreprovided a can 49 which seals off the stator chamber 50 against thefore-vacuum side in a vacuum-tight manner. The can 49 penetrates theslot 28 between the coils 23 with their yoke components 24 and therotating magnetic-ring set 8. Said can is therefore expediently made ofa non-magnetizable and electrically not well conducting material, carbonfibre reinforced plastic, for example.

[0029] At the side of the rotor the already detailed pipe-shapedreinforcement 14 is provided. It not only reinforces the ring set 8 butalso the motor's armature 48.

[0030] In order to compensate for tolerances, bearing 4 is adjustablevia adjustment screws 52 on which the carrier 16 for the stationary ringset 7 rests. Expediently the adjustment is performed such that therotating system is located axially in the unstable operating point.Axial control can be effected with minimum energy requirements aboutthis operating point.

[0031] Drawing FIGS. 4 to 7 depict different embodiments for the activemagnetic bearing 4. In the solution in accordance with drawing FIGS. 4(without magnetic field lines) and 5 (with magnetic field lines) eachfour magnetic rings form the rings sets 7 and 8. Only one coil 23 withits U-shaped yoke 24 is provided. The distance between the face sides ofthe U-limbs of yoke 24 corresponds approximately to the axial dimensionof one magnetic ring of ring set 8. For the purpose of attaining anoptimum interaction of the magnetic forces, the face sides of theU-limbs are located at the level of the centres of two neighbouringmagnetic rings of ring set 8, in the embodiment depicted at the level ofthe centres of the two middle magnetic rings.

[0032] In the embodiment in accordance with drawing 6, there is alsoonly provided one coil 23 with its yoke 24. The distance of the facesides of the limbs of the U-shaped yoke 24 facing the rings of the ringset 8 corresponds approximately to twice the axial dimension of amagnetic ring. Drawing FIG. 7 depicts a solution with five coils 23 andyokes 24. The ring set 8 has six magnetic rings. The face sides of the,in total six yoke limbs, are located approximately at the level of thecentres of the magnetic rings.

[0033] Between each of the rings of the magnetic-ring sets 7, 8 thereare located—as already detailed—annular spacing disks 31, whichdepending on the material have an influence on the formation of themagnetic field lines and/or the damping effect.

[0034] Expedient designs for the annular spacing disks 31, preferablyfor attaining a damping effect as well as supplementing the coatings ofthe magnetic rings are explained with reference to the embodiments ofbearing 3 depicted in the drawing FIGS. 8 to 10.

[0035] If the material for the annular spacing disks 31 consists of amaterial with good electrical conducting properties expedient forattaining a damping effect, then it may be expedient for the purpose ofimproving the damping effect to reinforce the circumferences of theannular spacing disks 31 at the point where the magnetic field entersinto the slot 9, for example increasing continuously outwards, and toadapt the shape of the magnetic rings to such circumferences. Thisembodiment is depicted in drawing FIG. 8. The reinforced circumferenceof the middle annular spacing disk 31 close to the slot is designated as54. In that the magnetic fields pass through more conducting material,the counterforces generated by the eddy currents and providing thedamping effect, increase.

[0036] In the embodiment in accordance with drawing FIG. 9, for example,the magnetic rings of ring set 5 are coated (coating 55) on all sides.At the side they have the function of spacing disks 31, so that when ofsufficient thickness of the coating 55 and suitably selected materials,said spacing disks will influence the magnetic field lines and/or have adamping effect. In addition it is achieved that the magnetic rings areprotected against aggressive gases. Such protection may also be attainedin that a sleeve 32 is provided, be it step-shaped as already detailedwith reference to drawing FIG. 2, or cylindrical as depicted in drawingFIG. 10 (ring set 5), for example.

[0037] The annular spacing disks 31 (resp. coating 55) of the magneticrings need(s) to be sufficiently thick to fulfil their/its purpose,particularly since also the desired stiffness of the bearing depends onthe thickness of the spacing disks. In medium size drag pumps athickness in the range from 0.25 to 1 mm has been found to be practical.

[0038] Moreover, the employment of spirally wound foil coils 23 has beenfound to be expedient since their space requirement is relatively small.

1. Machine (1), comprising a stator and a rotor (2) which is mountedwith radially stable and axially unstable magnetic bearings (3, 4),whereby the magnetic bearings (3, 4) each consist of concentricallyarranged magnetic ring sets (5, 6; 7, 8) in which the stationarymagnetic ring set (5 or 7) is situated inside and the rotating magneticring set (6 or 8) is situated outside as well as means (21, 23, 24, 27)for regulating the axial position of the rotor (2) wherein one (4) ofthe two magnetic bearings (3, 4) is itself equipped with the axialregulation means and at least one coil (23) is controlled by a positionsensor (21) and pole components (24) surround the outer magnetic ringset (8) of the axially regulated bearing (4).
 2. Machine in accordancewith claim 1, wherein the magnetic ring sets (5 to 8) consist of ringsmagnetised in the axial direction which are arranged over each otherwith changing poles, and where the ring sets (5, 6 resp. 7, 8) of abearing (3, 4) repel each other.
 3. Machine in accordance with claim 2,wherein the cross section of the pole components (24) is designed to beU-shaped and where the face sides of the U-limbs of the pole components(24) face the outer magnetic ring set (8).
 4. Machine in accordance withclaim 3, wherein the face sides of the U-limbs of the pole components(24) have a distance which corresponds approximately to the single ormultiple axial distance of a magnetic ring and where the coils/yokecomponents are so arranged that the face sides of the U-limbs arearranged at the level of the centers of the magnetic rings.
 5. Machinein accordance with claim 4 or 5, wherein one or several coil(s) (23)with one, resp. one each U-shaped pole component (24) is/are provided.6. Machine in accordance with one of the claims 2 or 5, wherein for nmagnetic rings of the outer ring set (8) n−1 coils (23) are providedwhich are each encompassed by an arrangement of U-shaped pole components(24) and where the limbs of the U-shaped yoke components (24) arelocated approximately at the level of the centers of the magnetic rings.7. Machine in accordance with one of the above claims, wherein thenumber of the magnetic rings of the two ring sets of a magnetic bearingdiffer.
 8. Machine in accordance with claim 7, wherein the number ofmagnetic rings of the rotating magnetic-ring set (6, 8) is greater thanthe number of magnetic rings of the stationary pair of magnetic rings(5, 7).
 9. Machine in accordance with one of the above claims, whereinthe magnetic rings of the ring sets (5, 6, 7, 8) are affixed inreceptacles, where as the receptacle for the magnetic rings of the outerring set (8) of the axially active bearing (4) a pipe-shapedreinforcement is employed which is affixed to a first section at therotating system (2) and with a second section that carries the magneticrings of the ring set (8).
 10. Machine in accordance with claim 9,wherein the reinforcement (14) also encompasses the armature (48) of adrive motor (46).
 11. Machine in accordance with claim 2 and one theclaims 3 to 10, wherein centrally arranged carriers (15, 16) areprovided for the stationary magnetic-ring sets (5, 7) where one of thecarriers (15, 16) is equipped with a central bore (19), where a shaftend (20) of the rotating system (2) penetrates the bore (19) and wherean axial sensor (21) is related to the unoccupied face side of the shaftend (20).
 12. Machine in accordance with claim 11, wherein one of thecarriers (15, 16) is axially adjustable.
 13. Machine in accordance withone of the above claims, wherein the diameters of the facingcircumferential surfaces of the rings of a pair of ring sets (5, 6 resp.7, 8) change in steps.
 14. Machine in accordance with one of the aboveclaims, wherein at least between a part of the magnetic rings of themagnetic ring sets (5, 6, 7, 8) there are located spacing ring disks(31) made of a non-ferrite material.
 15. Machine in accordance withclaim 1, wherein the material of the spacing disks (31) has a highelectrical conductance.
 16. Machine in accordance with claim 15, whereinthe circumferences of the spacing disks (31) close to the slot arereinforced.
 17. Machine in accordance with claim 14, 15 or 16, whereinthe magnetic rings are encapsulated and where the layers of theencapsulation have the function of the spacing ring disks (31). 18.Machine in accordance with claim 14, 15 or 16, wherein a sleeve (32) isassigned to the circumferential surfaces of the magnetic rings close tothe slot.
 19. Machine in accordance with one of the above claims,wherein it is designed as a drag pump (1) and where the passive bearing(3) is located at the high-vacuum side, the axially active bearing (4)at the fore-vacuum side.
 20. Machine in accordance with claim 19,wherein a drive motor (46) with a can (49) is provided and where the can(49) penetrates the slot (28) of the axially active bearing (4).